In the presence of a 24 hour cell-free and Con A-free supernatant of spleen cells activated by Con A (Con A supernatant) isolated mouse B lymphocytes respond in vitro at low cell density to anti-Ig beads (insoluble rabbit F(ab')2 anti-mouse Fab covalently coupled to polyacrylamide beads) by sustained proliferation and differentiation to polyclonal IgM and IgG secretion. The response is comparable in magnitude to the LPS response. Ig secretion is completely dependent on the Con A supernatant, even at higher cell densities at which beads alone give a good proliferative response. The supernatant alone has only a relatively small, primarily differentiative effect. The Con A supernatant can be added a day after the start of culture without changing the kinetics of the response. Cells from newborn mice, adult CBA/N mice, and adult male (CBA/N female x Balb/c male) Fl mice to respond to anti-Ig beads plus the Con A supernatnat. These experiments suggest that redistribution of surface Ig by the anti-Ig beads delivers a limited proliferative signal to a subset of B lymphocytes, but that sustained cell division and differentiation to Ig secretion by anti-Ig activated B lymphocytes require soluble factors produced by activated T lymphocytes and/or T lymphocyte-activated accessory cells. We propose to exploit this system to study early events in B cell activation, particularly to distinguish signals required to initiate DNA synthesis from signals required to initiated differentiation to high rate Ig synthesis and secretion, and to begin to study the surface molecules, surface events, and intercellular signals which regulate these processes. Techniques will include: reverse hemolytic plaque assay, isotype incorporation, intracellular immunofluroescence, autoradiography, isolation of B cell subsets as hapten-sandwich rosettes, and blast cell isolation.